1. Field of the Invention
The present invention relates to the sterilization of containers and particularly pot-like containers destined to receive foodstuffs. More specifically, this invention is directed to apparatus for applying a coating of a sterilizing agent to the interior surfaces of containers, such as plastic pots, in which dairy products are to be packaged for subsequent retail sale. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
Methods and apparatus for sterilizing containers, wherein a uniform coating of an atomized liquid sterilizing agent is deposited on the internal surfaces of the container, are known in the electrostatic art. Examples of prior art sterilizing appliances for use in treating open-topped plastic containers may be seen from U.S. Pat. No. 4,680,163 which is assigned to the assignee of the present invention and which is incorporated herein by reference. The prior art apparatus comprises an aerosol generator with an ultrasonic transducer that operates at frequencies in the MHz range. The aerosol generator produces droplets of a liquid sterilizing agent which are entrained in a controlled stream of carrier gas. The prior art apparatus further comprises a deposition arrangement in which the entrained droplets of sterilizing agent are charged by directing them into a corona discharge. The charged droplets are subsequently deposited onto the surfaces of the container under the influence of an electrostatic field. The apparatus by which the charging and subsequent uniform deposition of the droplets of sterilizing agent is achieved comprises a deposition head which extends into the container to be sterilized. This deposition head has a centrally located, pointed electrode that is energized by a high voltage generator. The deposition head further comprises an outer electrode which surrounds the container and an auxiliary electrode which also extends into the container and is coaxial with the pointed electrode and outer electrode although not coextensive in length therewith. The electrostatic field which causes the charged droplets to travel to and be deposited upon the container wall is established between the outer electrode, which is typically at ground potential, and the pointed electrode and auxiliary electrode.
While the above-described prior art sterilizing apparatus and the method encompassed by its operation have worked well, a number of areas in which functional improvement would be desirable have been observed. Thus, it has been discovered that the deposition of charged droplets of sterilizing agent on the wall of a non-conductive container being treated may result in a charge build up which effects the corona discharge being utilized to charge the droplets and, in fact, may cause the corona discharge to be extinguished. Condensation of the atomized sterilizing agent on surfaces of the apparatus has also sometimes presented an operational problem.